Masayuki Ohmori

HAR1 mediates systemic regulation of symbiotic organ development

Symbiotic root nodules are beneficial to leguminous host plants; however, excessive nodulation damages the host because it interferes with the distribution of nutrients in the plant. To keep a steady balance, the nodulation programme is regulated systemically in leguminous hosts. Leguminous mutants that have lost this ability display a hypernodulating phenotype. Through the use of reciprocal and self-grafting studies using Lotus japonicus hypernodulating mutants, har1 (also known as sym78), we show that the shoot genotype is responsible for the negative regulation of nodule development. A map-based cloning strategy revealed that HAR1 encodes a protein with a relative molecular mass of 108,000, which contains 21 leucine-rich repeats, a single transmembrane domain and serine/threonine kinase domains. The har1 mutant phenotype was rescued by transfection of the HAR1 gene. In a comparison of Arabidopsis receptor-like kinases, HAR1 showed the highest level of similarity with CLAVATA1 (CLV1). CLV1 negatively regulates formation of the shoot and floral meristems through cell–cell communication involving the CLV3 peptide. Identification of hypernodulation genes thus indicates that genes in leguminous plants bearing a close resemblance to CLV1 regulate nodule development systemically, by means of organ–organ communication.

A Lotus basic leucine zipper protein with a RING-finger motif negatively regulates the developmental program of nodulation

The developmental program of nodulation is regulated systemically in leguminous host species. A mutant astray (Ljsym77) in Lotus japonicus has lost some sort of its ability to regulate this symtem, and shows enhanced and early nodulation. In the absence of rhizobia, this mutant exhibits characteristics associated with defects in light and gravity responses. These nonsymbiotic phenotypes of astray are very similar to those observed in photomorphogenic Arabidopsis mutant hy5. Based on this evidence, we predicted that astray might contain a mutation in the HY5 homologue of L. japonicus. The homologue, named LjBzf, encodes a basic leucine zipper protein in the C-terminal half that shows the highest level of identity with HY5 of all Arabidopsis proteins. It also encodes legume-characteristic combination of motifs, including a RING-finger motif and an acidic region in the N-terminal half. The astray phenotypes were cosegregated with LjBzf, and the failure to splice the intron was detected. Nonsymbiotic and symbiotic phenotypes of astray were complemented by introduction of CaMV35S:LjBzf. It is noteworthy that although Arabidopsis hy5 showed an enhancement of lateral root initiation, Lotus astray showed an enhancement of nodule initiation but not of lateral root initiation. Legume-characteristic combination of motifs of ASTRAY may play specific roles in the regulation of nodule development.

Screening for the target gene of cyanobacterial cAMP receptor protein SYCRP1

The target genes for SYCRP1, a cyanobacterial cAMP receptor protein, were surveyed using a DNA micro‐array method. Total RNAs were extracted from a wild‐type strain and a sycrp1 disruptant of Synechocystis sp. PCC 6803, and the respective gene expression levels were compared. The expression levels of six genes (slr1667, slr1668, slr2015, slr2016, slr2017 and slr2018) were clearly decreased by the disruption of the sycrp1 gene. The data suggest that slr1667 and slr1668 constitute one operon and the other four genes constitute another operon. Transcription start points for the first genes of these putative operons, which are slr1667 and slr2015, were determined by primer extension experiments. Gel mobility shift assays and DNase I footprint analyses were carried out to explore the binding of SYCRP1 to the putative promoter regions of slr1667 and slr2015. SYCRP1 bound to the specific site in the 5′ upstream region of slr1667 from positions –170 to –155 relative to the transcription start point, while it did not bind to the 5′ upstream region of slr2015. It was concluded that SYCRP1 regulates the expression of the slr1667 gene directly by binding to a specific site in its promoter.

Regulation of cAMP-mediated photosignaling by a phytochrome in the cyanobacterium Anabaena cylindrica.

Changes in cellular adenosine 3′,5′‐cyclic monophosphate (cAMP) content induced by monochromatic light of various wavelengths were determined in the cyanobacterium Anabaena cylindrica. Irradiation with monochromatic red light caused a rapid decrease in cAMP content. In contrast, far‐red light caused a rapid increase in its content. The effects of red and far‐red light were reversible, suggesting the involvement of a prototype phytochrome as the photoreceptor for cAMP‐mediated light‐responsive signal transduction.

Targeted Mutagenesis of the 45-Kda Protein Inactivates Nitrate Transport in Synechococcus PCC 7942

Nitrate is taken up into cyanobacterial cells by an active transport system (Flores et al. 1983; Tischner and Schmidt 1984; Lara et al. 1987). Little is known about the mechanism of this transporting system and a protein(s) involved in the transport is not yet identified. Such protein(s) would be located in the cytoplasmic membrane and its synthesis might be suppressed when cells do not utilize nitrate for their growth. We found that ammonium-grown cells of Synechococcus PCC 7942 lack the 45-kDa protein, a major component of the cytoplasmic membrane in nitrate-grown cells (Omata and Ogawa 1987). In order to elucidate the function of the 45-kDa protein, we cloned the gene (nrtA) encoding this protein and constructed an insertional mutant (M45) of Synechococcus that cannot synthesize the 45-kDa protein. Biochemical and physiological characterizations of the mutant indicated that the 45-kDa protein plays an essential role in the nitrate-transporting system.